View source for Opto-electronic oscillator

{{short description|Circuit which produces an optical or electronic sine wave signal}}
[[File:OEO_ingle-loop opto-electronic oscillator_BostjanBatagelj.jpg|thumb|right|Single-loop opto-electronic oscillator. LD is a [[laser diode]]. PD is a [[photodetector]].]]

In [[optoelectronics]], an '''opto-electronic oscillator''' ('''OEO''') is a [[electric circuit|circuit]] that produces a repetitive electronic [[sine wave]] and/or [[modulated]] optical continuous wave signals.

An opto-electronic oscillator is based on converting the continuous light energy from a [[pump laser]] to [[radio frequency]] (RF), [[microwave]] or [[mm-wave]] signals. The OEO is characterized by having very high [[quality factor]] (Q) and [[Stability (mathematics)|stability]], as well as other functional characteristics that are not readily achieved with [[electronic oscillator]]s. Its unique behavior results from the use of electro-optical (E/O) and [[photonic]] components, which are generally characterized with high efficiency, high speed, and low [[Dispersion (optics)|dispersion]] in the microwave frequency regime.

In an OEO, the [[phase noise]] of the oscillator does not increase with the frequency that is subject to other implementations by [[electronic oscillator]]s such as [[Crystal oscillator|quartz crystal oscillator]]s, [[dielectric resonator]]s, sapphire resonators or air-dielectric resonators.

== History ==
OEO was introduced in the early 1990s.<ref name=ref1>R.T. Logan, L. Maleki, M. Shadaram, "Stabilization of oscillator phase using a fiber-optic delay-line", in Proc. 45th Annu. Symp. on Frequency Control, pp. 508-512, May 1991</ref>

Since then, the device’s key properties have been continuously improved.

== Operation ==
Most OEOs utilize the transmission characteristics of an [[optical modulator]] together with a [[fiber-optic]] [[Analog delay line|delay line]] to convert light energy into stable, [[Spectral purity|spectrally pure]] RF/microwave reference signals. Light from a [[laser]] is introduced into an electro-optic (E/O) modulator, the output of which is passed through a long optical fiber and detected with a [[photodetector]].  The output of the photodetector is [[Amplifier|amplified]] and filtered and fed back to the electric port of the modulator.  This configuration supports self-sustained oscillations, at a frequency determined by the fiber delay length, the bias setting of the modulator, and the [[Bandpass filter|band pass]] characteristics of the filter.  It also provides for both electric and optical outputs.  The conditions for self-sustained oscillations include coherent addition of partial waves each way around the loop and a [[loop gain]] exceeding losses for the circulating waves in the loop.  The first condition implies that all signals that differ in [[phase (waves)|phase]] by some multiple of 2π from the fundamental signal may be sustained.  Thus the oscillation frequency is limited only by the characteristic frequency response of the modulator and the setting of the filter, which eliminates all other sustainable oscillations.  The second condition implies that, with adequate light input power, self-sustained oscillations may be obtained without the need for the RF/microwave amplifier in the loop.

Chip-scale OEOs utilize [[whispering gallery mode]] optical resonators rather than a delay line. Whispering gallery mode optical resonators are axially symmetric [[dielectric]] structures ranging in size from tens of [[micrometers]] to a few [[millimeters]], and can trap light in a small volume. The modes are solutions of [[Maxwell's equation]] and represent waves that propagate close to the surface of the resonator structures, along the perimeter.<ref>Ilchenko, V.S., Miniature oscillators based on optical whispering gallery mode resonators, Frequency Control Symposium, 2008 IEEE International, {{ISSN|1075-6787}}</ref>

== Theory ==
[[Quality factor]] (Q) of OEO is determined from a resonator's centre frequency ''f''<sub>0</sub> and [[group delay]] {{mvar|&tau;}}
: <math>Q = { \omega_0 \tau \over 2 } = \pi f_0 { n L \over c_0 },</math>
where ''n'' is the refractive index, ''L'' is the optical fibre length and ''c''<sub>0</sub> is the speed of light in vacuum.

== Uses ==
A high-performance OEO is a key element in a variety of applications, such as 
* modern radar technology,
* aerospace engineering,
* satellite communication links,
* navigation systems,
* precise metrological time and frequency measurements,
* reference clock distribution,<ref name=ref2>Jurij Tratnik, Primoz Lemut and Matjaz Vidmar, [http://www.midem-drustvo.si/Journal%20papers/MIDEM_42(2012)2p115.pdf "Time-transfer and synchronization equipment for high-performance particle accelerators"], Informacije MIDEM, Vol.42, no.2, pp. 115-122, 2012</ref> and
* high-bitrate, optically supported, communication wireless links, including [[radio over fiber]] technology.

== See also ==
* [[Delay-line oscillator]]

== References ==
{{reflist|2}}

== External links ==
* [https://ieeexplore.ieee.org/document/7193351/ Key properties and design issues for an opto-electronic oscillator], 2015
* [http://nasatech.com/Briefs/Apr01/NPO20951.html Highly Oblate Microspheroid as an Optical Resonator]
* [http://spie.org/x39063.xml?ArticleID=x39063 Crystalline resonators add properties to photonic devices], 2010 February 17

[[Category:Electronic oscillators]]